In the machining area and particularly for electronic interconnects, protective coatings are widely used to provide protection to substrates in corrosive environments. The demand on high-performance protective coatings continue to grow, as advances are being made in cutting-edge areas such as outer-space and deep-sea exploitation, defense weaponry, and nanotechnology concerning nano machines. Good-quality coatings are also in need in the consumer appliances and electronics industries, so are efficient and reliable processes for applying such coatings.
Many types of coating materials and a range of coating techniques have been used and reported in the material, electronics, chemical engineering, and other related industries. See, generally, B. Popov (2004) Electro Deposition of Alloys & Composites with Superior Corrosion and Electrical Properties, Plating and Surface Finishing; J. Dini, 1992, Electrodeposition: The Material Science of Coatings and Substrates, p. 316, Noyce Publication; E W. Brooman (2000) Corrosion Behavior of Environmentally Acceptable Alternatives to Cadmium & Chromium Coating, Metal Finishing, pages 42-50. Common coating techniques include among other things electro plating, electro-less plating, vacuum deposition, sputtering, evaporation, and thermal spray or High Velocity Oxygen Fuel (HVOF). Of these techniques, electro plating and electro-less plating require relatively low investment, are relatively easy to operate, and thus are favored by many users.
The standards for high-performance coatings include many aspects, chief among which is corrosion resistance. The American Society for Testing and Materials (ASTM) International is the industry organization responsible for establishing and maintaining such standards. Founded in 1898 and formerly known as ASTM, ASTM International provides a global forum for the development and publication of standards for materials, products, systems, and services (“ASTM Standards”). The adoption of ASTM Standards is generally voluntary and by consensus; however, many ASTM Standards have become widely accepted as the industry standards. In more than 130 industry areas, ASTM Standards serve as a basis for research, manufacturing, testing and quality control, commercial transaction, and regulatory activities.
Specifically with respect to corrosion, ASTM Committee G01 on Corrosion of Metals, a committee formed in 1964, has jurisdiction over more than 70 standards, published in the Annual Book of ASTM Standards. These standards have played, and continue to play, an important role in all industries concerned with the problem of corrosion. Examples of such corrosion problem include corrosion of nuclear materials, atmospheric corrosion, corrosion in computers and other electronic devices, corrosion in natural waters, corrosion in soils, and corrosion of reinforcing steel. Laboratory corrosion tests and other similarly controlled tests have been designed and used to measure corrosion, and evaluate corrosion resistance qualitatively and/or quantitatively.
One principal corrosion test is the salt spray test pursuant to ASTM-B 117, and another principle corrosion test is the sulfur dioxide gas spray test pursuant to ASTM-G 85. Both standards will be discussed in more detail below. As quality and performance requirements become more and more demanding especially for materials in highly specialized fields such as outer space and deep sea operations, the need for protective coatings to meet high standards becomes increasingly pressing. Few, if any, commercially available coating today is capable of withstanding at least 1000 hours of salt spray pursuant to ASTM-B 117, and none is capable of withstanding at least 336 hours of sulfide dioxide gas spray pursuant to ASTM-G 85.
For example, cadmium, or cadmium over nickel (a.k.a. Olive Drab or Cd/Ni), has been universally accepted as a coating of choice where strong corrosion resistance to neutral salt spray and a certain level of inherent lubricity are required. Many alternative metals and metal alloys (including zinc alloys and tin alloys) have been evaluated, and none has yet performed as well as Cd/Ni. However, in the aggressive pH 2.5 condition of sulfur dioxide gas, even Cd/Ni coating fails miserably.
In addition to corrosion resistance and lubricity, the desirability of a protective coating is also dictated by its potential environmental and/or health impact. For example, cadmium has been identified as a carcinogen, and efforts have been underway to find a replacement for cadmium or the Cd/Ni coating described above. See, id., E W. Brooman, Mental Finishing. Another fold of reason for replacing Cd/Ni coating is that it contains hexavalent chromium—a substance that lends Cd/Ni its distinctive olive color and enhances its resistance to salt spray. Hexavalent chromium is deemed carcinogenic by the U.S. Environmental Protection Agency (EPA), and restricted in Europe pursuant to a European Union Directive that has taken effect on Jul. 1, 2006. See, Restriction of Hazardous Substances in Electrical and Electronics Equipment (RoHS) Directive (2002/95/EC). Promulgated in Europe, this RoHS Directive has worldwide implications. It restricts, or aims to eliminate, the use of cadmium, chromium, and other chemicals that are deemed environmental and/or health hazards.
Therefore, there is a need for protective coatings to replace cadmium where corrosion resistance and natural lubricity of cadmium are required, and where the coatings remain electrically conductive. There is also a related need for protective coatings capable of withstanding prolonged exposure to both acidic sulfur dioxide gas spray and neutral salt spray. There is a further need for methods, processes, or techniques of applying such protective coatings to substrates of varied nature and properties.